Insensitive munitions (IMs) are desirable alternatives to historically used formulations, such as 2,4,6-trinitrotoluene (TNT), due to their insensitivity to unintended detonation. The insensitive munition IMX-101 is a mixture of 2,4-dinitroanisole (DNAN), 3-nitro-1,2,4-triazol-5-one (NTO), and nitroguanidine (NQ). Environmental releases of munitions may be from production wastewaters or training; these munitions may be exposed to Ultraviolet light. Therefore, it is useful to understand the relative toxicity of IMX-101 and its constituents both before and after photo-degradation. The intent of the present study was to generate relative hazard information by exposing the standard ecotoxicological model Ceriodaphnia dubia to each IM constituent individually and to IMX-101 before and after the exposure solution was irradiated in a UV photo-reactor. Without photo-degradation, DNAN was more toxic (lethal median concentration, or LC50=43mg/L) than the other two constituents and it contributed predominately to the toxicity of IMX-101 (LC50=206mg/L) based on toxic units. Toxicity was observed only at high levels of NQ (LC50=1174mg/L) and pH-adjusted NTO (LC50=799mg/L). The toxicity of IMX-101 is lower than literature reported values for TNT toxicity. Photo-degradation efficiency was greater at lower IM concentrations. The observed degradation was greatest for NQ (42-99%), which in turn corresponded to the greatest relative increase in toxicity (100- to 1000-fold). More modest percent degradation (4 to 18%) and increases in photo-toxicity (2 to 100-fold) were observed for NTO and DNAN. Photo-degraded NQ products were the predominant source of toxicity of photo-degraded IMX-101. Future work involves research to enable analytical and computational confirmation of the specific degradation compounds inducing the observed photo-enhanced toxicity. This article is protected by copyright. All rights reserved.